Linear accelerator system for stable pulsing at multiple dose levels

ABSTRACT

A linac-based X-ray system for cargo scanning and imaging applications uses linac design, RF power control, beam current control, and beam current pulse duration control to provide stable sequences of pulses having different energy levels or different dose.

RELATED APPLICATION

This application is related to and claims the benefit of U.S. patentapplication Ser. No. 14/192,864, filed on Feb. 27, 2014, now U.S. Pat.No. 9,622,333, and also is related to and claims the benefit of U.S.patent application Ser. No. 14/634,361, filed Feb. 27, 2015, now U.S.Pat. No. ______, both of which are incorporated herein by reference forall purposes.

FIELD OF THE INVENTION

The present invention relates generally to X-ray devices, and moreparticularly relates to linac-based scanning systems and methods able togenerate pulses of different energies and doses.

BACKGROUND OF THE INVENTION

Linear accelerator (“linac” hereinafter) X-ray generating systems havebeen in use in the medical environment for a number of years. Morerecently, such systems have begun to be used in the industrialenvironment, particular for cargo scanning. To distinguish the materialsinside a cargo container, X-ray pulses of different energies have beenused. Energies in the range of 2 MeV to 10 or more MeV have beenproposed in the literature, and commercial devices offering energies atapproximately 4 MeV and 6 MeV are commercially available. In manyinstances, both medical and industrial linac X-ray sources are excitedby RF sources operating at or near the S-band, roughly 2.998 GHz. Ineach case, the linac accelerates a stream of electrons in conjunctionwith RF excitation, and the linac must be designed such that itsresonance can be matched by the frequency of the RF source. Once theelectrons have been sufficiently accelerated, if X-rays are desired,they strike a target, such as tungsten, resulting in the emission ofhigh energy X-rays that can be used for medical treatment, materialsprocessing, scanning of cargo, and other applications. Thesemega-electron-volt (MeV) X-ray applications have provided significantbenefits in many fields.

The most prevalent medical and industrial linacs are resonantstructures, and require an RF excitation source, typically either apulsed magnetron or a pulsed klystron. To couple the power from an RFexcitation source into a linac, for the purpose of acceleratingelectrons, the frequency of the RF source output must be adequatelymatched to the resonance frequency of the linac structure. Standing wavelinear accelerators are somewhat more sensitive to the accuracy of theexcitation frequency than are traveling wave accelerators, yet both aresensitive. In addition, a given linac's optimum excitation frequency istypically sensitive to temperature of the linear accelerator, as is wellknown. The output frequency of the RF excitation source can also changewith environmental conditions such as temperature, and AFC (automaticfrequency control) circuits are often used to maintain a good matchbetween the RF source and the linac.

The maximum electron energy, and thus the resulting X-ray energy thatcan be obtained from a given linac structure is dependent upon the peakpower coupled into it from the RF source, and also dependent upon thebeam current within the accelerator at the time the RF power is applied,using known relationships. In addition, the Q of the linac can affectthe performance of the system. For most linac-based systems, a high Qhas been deemed desirable, typically between 5000 and 10,000.

In industrial applications such as cargo scanning, X-ray pulsing permitsimages of the contents of a container to be created without opening thecontainer. The high energy, MeV level X-rays from linac systems allowadequate penetration through large containers and their typicalcontents. The image of the contents of a container is typically acomposite of a large number of scans, or image “slices” at differentenergies, in some cases 1000 to 10,000 or more such slices. As anexample, a commercial cargo scanning system may pulse a linac at 100 to400 X-ray pulses per second, as a truck passes through a scanner, andthe images from each of those pulses are then composited to create thecompleted image. In some prior art cargo scanning operations, imagingsystems that use linacs can scan between 24 and 150 trucks per hourdepending upon the mode.

Different X-ray energies, such as 4 MeV and 6 MeV, are useful in cargoscanning because it permits the materials in the container to bedifferentiated. By comparing the images taken at 4 MeV with those takenat 6 MeV, steel can be distinguished from uranium, as just one example.Likewise, organic material, aluminum, lead, and so on, can bedistinguished.

However, it is impractical to take a first scan of a container restingon the bed of a truck at a first X-ray energy, and then take a second,later scan of that same container at a second X-ray energy. Instead, itis more practical to interleave pulses of the different frequencies, sothat a scan of both energy levels is taken in a single pass, for exampleby means of interleaved pulses of two energies in an ABABABAB pattern.This interleaving, however, can present challenges to a linac systemsince the performance of a linac with its resonant structure is highlydependent on a good frequency match. Some well known techniques formodifying the input parameters to a typical 6 MeV linac system, in orderto obtain a 4 MeV output, involve either reducing RF power to reduce themaximum possible acceleration for electrons in the linac, or increasingthe beam current to provide beam loading that ultimately reduces themaximum acceleration that each electron experiences, or a combination ofthese two effects. These techniques have been sufficient when the timebetween energy changes is long, such as seconds or many seconds, buthave been less effective when the time between energy changes is afraction of a second. It is also possible to intentionally detune theresonance frequency of the RF source with respect to the linac—this hasthe effect of lowering the amount of RF power coupled into the linac,reducing the total possible acceleration for electrons. However,detuning can cause instabilities in performance, and is difficult to doin a fraction of a second if a mechanically-tuned magnetron is the RFsource. A challenge with applying prior art techniques for changinglinac energy on a rapid time scale in an ABABABAB fashion can be theundesired corresponding detuning of the RF source. For example, inaddition to intentional tuning, reducing or increasing RF power from amagnetron typically results in a change in frequency of the RF output.If the frequency change is large enough, the linac's resonance is nolonger matched to the frequency of the RF excitation pulse, and thesystem fails to operate. A pulse-to-pulse variation of RF amplitude orvariation in RF frequency in a detuned condition will cause a greaterchange in linac output performance than operation at peak or tunedcondition.

While the terms “4 MeV” and “6 MeV” are commonly used in the art, thoseterms typically refer to the peak energies of the X-ray pulses, and theaverage energy can be less. These terms will be used herein with thatsame understanding.

Because of the sensitivity of linac-based systems to changes infrequency, the prior art has generally not been able to provide a fullyoptimized train of stable, interleaved pulses of different energies suchas an ABABABABAB (etc.) pattern, especially where the energy must bechanged rapidly, from one pulse to the next, at a 2.5-millisecond basisor shorter such as required for a pulse rate of 400 or more pulses persecond, and especially when that rapid change is done with a magnetrondriven system

While magnetrons do not offer some of the advantages of klystrons,magnetrons such as the MG5193, MG6090, and MG7095 from supplier e2V havetypically been used as RF sources for medical and cargo scanningapplications. Similar magnetrons are available from National Japan RadioCorporation (NJRC). Unlike klystrons, magnetrons are not amplifiers, andthe output frequency of a magnetron is adjusted by a mechanical tuner.This limits the ability to rapidly switch a magnetron system betweenhigher and lower energy levels while still maintaining a frequency matchwith the linac, because the magnetron frequency will shift upon a powerchange and a mechanical tuner simply cannot be moved rapidly enough tosupport a 2.5 millisecond or shorter period between pulses. The dutycycle of such devices is typically low, for example approximately 0.1%for the MG5193, MG6090, and MG7095, such that pulse durations of a fewmicroseconds, for example 2.5 microseconds, can be generated at up to400 pps. For an MG5193, 4.5 turns of the mechanical tuner allow forabout nine megahertz of tuning range. One drawback of a magnetron isthat its mode and stability can become unfavorable if the magnetron isoperated at a peak power too much different from its optimum or maximumpeak power. The MG5193 can operate between about 1.5 MW peak and about2.6 MW peak. The MG6090 and MG7095 can provide higher peak poweroutputs, such as 3 MW of peak power.

It is possible to change the power output of a magnetron rapidly throughthe use of a conventional high voltage capacitor-charging power supplytogether with a pulse forming network/modulator. With such a technique,interleaved high voltage (“HV”) pulses can be supplied to the magnetronin an ABABABAB sequence. The result is that the RF power output of themagnetron can be varied rapidly as well, and also in an alternatingfashion.

However, as noted before, changing the power output of the magnetronalso causes a change to the output frequency of the RF pulse, such thatthe output frequency of the magnetron can be a mismatch for the linac.For example, the MG5193 and similar magnetrons can have a frequencyshift of about 10 kHz per ampere, and they are typically driven ataround 100 amps or more. Changing this current by many amperes may causea significant detuning with respect to the resonance of a given linac.While AFC circuits can compensate for long term changes in frequency,such circuits are not intended to compensate for instantaneouspulse-to-pulse changes such as occur with a magnetron is driven with analternating sequence that varies by several amps. As a result, suchapproaches can result in instability of the magnetron-linac system, andthere has been a need for a system and technique for maintains stabilitywhile permitting the use of interleaved pulses of different RF powers togenerate an interleaved pattern of X-ray pulses of different energies.

In addition to the challenges to prior art systems that result fromvariations in RF matching between the output frequency of a magnetronand the resonant frequency of the linac, the prior art has also hadchallenges in maintaining a consistent dose from pulse to pulse wheninterleaved pulses of different energies are generated. While dosecontrol is known in medical systems, the need for ABABABAB sequences ofdifferent energies is not typically found in medical X-ray systems. Dosecontrol in prior systems typically involves either changing the RF peakpower coupled into the linac, or through beam loading, which involveschanging the peak electron beam current into the linac, or both. Forexample, it is well known that increasing the peak beam current into anaccelerator will reduce the energy of the electrons leaving the linac.The value of the beam current can be controlled by controlling theelectron gun, but, if the maximum energy of the electron beam is change,then the dose rate will also change absent taking steps to prevent thatchange. For example, using beam loading to change the X-ray energy from6.5 MeV to 5 MeV can change the dose in some systems by a factor of two.However, for the use of X-rays to differentiate among materials, asrequired for cargo scanning applications, such changes in dose per pulseare undesirable. Instead, it is preferable that both energy and dose becontrolled on a pulse-by-pulse basis even where the pulses are atdifferent energies, such as energies that are different by more than 1MeV.

While dose control in medical applications can be achieved throughchanges in repetition rate, this is not desirable for cargo scanningapplications which depend upon having consistent repetition rates. As aresult, there has been a need for a cargo scanning system which offersdose control on a pulse-by-pulse basis while offering stable sequencesof interleaved pulses of different energies.

A further problem in cargo scanning applications is the intermittentnature of the operation of such scanning systems. The X-ray emission ina cargo scanning system is typically turned off after each scan iscompleted. For example, a first truck, carrying a container, can bescanned, and then the scanner is turned off. A next truck carrying acontainer arrives a few seconds later, or a minute, or some otherindeterminate time period. As noted previously, the operating frequencyof the linac changes with temperature. It is well known that the linacheats up during scanning, which requires that the output of themagnetron be adjusted to maintain a good match with the linac. While AFCcircuits, which typically rely on feedback of the forward and reflectedpower of the linac, can maintain that good match when the linac systemis being pulsed, there is no such feedback when the scanner is off andthe linac starts to cool. As a result, typical prior art systems usingconventional AFC may leave the tuner of the magnetron in the sameposition it was in when the scanner was turned off. Depending upon thefrequency drift of the linac versus the RF source during the “off”period and the mismatch between RF source frequency and linac resonance,the system may simply not operate when scanning is restarted, or, morecommonly, the energy output and/or dose output of the linac system willbe less than intended. While this affects only the first few pulses, or10 pulses or more after scanning is re-initiated, before the AFC circuitcan achieve a good match, this lack of consistency in output energyand/or dose can affect the quality of the resulting images, and istherefore undesirable.

One prior art technique involves turning on the RF source to the linacin advance of enabling any beam current from the electron gun of thelinac, to perform a partial warm-up of the linac beamline by the RFsource prior to the electron beam triggering. In another approach, atypical AFC is used during the RF-only partial warm-up period, with anoffset added to the tuning in advance of any expected electron beamtriggering.

These warm-up approaches have the significant disadvantage ofpotentially generating some amount of X-rays even when the electron gunis not being pulsed. Such X-rays can be generated by virtue of the highelectric fields in the RF-pulsed linac pulling electrons from theelectron gun and cavity walls. Another disadvantage is the consumptionof average power that is not used for the generation of X-rays. Theseapproaches are inefficient and therefore undesirable.

As a result, there has been a need for a linac-based X-ray scanningsystem that provides consistent pulse energies even for the initialpulses of a sequence despite intermittent operation and “off” periods ofindeterminate duration.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned limitations of theprior art by providing a stable sequence of interleaved pulses ofdifferent energies, while at the same time providing consistent andprecise pulse-to-pulse dose control.

In one aspect of the present invention, a series of X-ray pulses of atleast two different desired energy levels is created by providing an RFpower generator, typically a magnetron, which supplies to a particleaccelerator such as a linac RF pulses of at least two differentcontrollable powers at two different corresponding rf frequencies. The Qof the linac is designed to be sufficiently low that the different RFfrequencies of the pulses from the RF power generator remain within theresonance bandwidth of the linac. At least one gun driver supplies atleast two different controllable current pulses to the electron gun ofthe linac, with the pulses of each different current synchronized tooccur within an envelope created by an RF pulse of a correspondingpower. The resultant electron pulse from the electron gun of the linacis accelerated until it strikes a target, for example tungsten, togenerate an X-ray pulse at one of the desired energy levels. The dose ofeach pulse is adjusted by altering the duration of the current pulse offrom the electron gun, which permits the X-ray pulses of differentenergy levels to supply the same dose per pulse, or different dosesadjusted to a desired ratio.

In a related aspect, in some instances it is desirable to scan a firstportion of an object or a vehicle using pulses of a first dose, and thenscan a second portion of the object or vehicle using pulses of a seconddose. For example, when scanning a truck, the driver is frequently inthe cab of the vehicle during the scan. It is desirable in at least someinstances to scan the cab where the driver is located with pulsessufficient to provide good imaging, without exposing the driver to thesame dose as is desirable for scanning the trailer. In an embodiment ofthe present invention, a pulse train of lower dose can be generatedwhile scanning the cab, and a pulse train of higher dose can begenerated while scanning the trailer. The pulse train of lower dose canbe alternating energy levels, such as ABABABAB, or can be a sequence ofpulses of a single energy level, such as AAAAAA or BBBBBB, or can be asequence of pulses having a different energy, such as CCCCCC, or can beany combination of these. In a still further related aspect, it will beappreciated that pulse trains of alternating energies can comprise morethan two energy levels as, for example, ABCDABCDABCDABCD, or anycombination of energy levels made desirable by the specificimplementation as long as the various pulses are consistent with thebandwidth of the linac.

In some embodiments comprising a single energy pulse but with higher andlower dose modes, a single grid gun driver can be implemented tosimplify design.

In a different aspect of the invention, compensation is provided for thechanges in performance in both the linac and the RF power generator asthe result of thermal and other influences caused by intermittentoperation. For example, in a cargo scanning application, the linacsystem is operated to generate X-rays during the period that a cargocontainer passes through the scanner. The scanning function is thenturned off, for example by disabling power to the RF power generator,until the next container is moved into position for scanning. Thescanning function is then restarted. The “off” period is of uncertainduration because it depends upon when the next container is properlypositioned. As a result, the performance parameters of both the RF powergenerator and the linac can vary significantly, to the point that amismatch between the frequency of the RF generator pulses and the linacresonance can occur which may prevent stable system operationaltogether.

To prevent such mismatches resulting from the intermittent operation ofthe linac system, the present invention provides a system and method foradjusting the frequency of the RF power generator during periods whenthe scanning function is disabled, such that, when scanning isre-enabled, the frequency of the RF power generator has been adjusted tomaintain a substantial match with the changed resonance of the linacresulting at least in part from the cooling of the linac while scanningwas disabled. In addition, in some embodiments, ambient temperature issensed and incorporated into a cool down compensation system and method.In at least some embodiments, the compensation results in the dose ofthe first pulse after re-enabling being within ten percent of theaverage dose during the period when scanning is enabled, and, dependingupon the embodiment, can be within two percent or one percent of theaverage dose.

Depending upon the embodiment, the RF power generator can be either amagnetron or a klystron. In those embodiments using a magnetron, thefrequency of the magnetron is adjusted by mechanically adjusting thetuner with a stepper motor, with the AFC causing motion of a number ofsteps from the “home” setting, where the home setting is the optimizedsetting of the tuner to match the RF output frequency with the resonanceof the linac when the linac is operated at a sufficiently low repetitionrate that thermal influences do not greatly change its resonance. Aftera period of operation, the AFC will move the tuner to an optimumposition for linac performance. During an “off” period of indeterminateduration, the cool down compensation system and method moves the tunerin the absence of feedback in order to maintain a match between RFsource frequency upon restarting pulsing, and that of the linac. In apreferred embodiment, the number of steps of tuner correction maycorrelate to the duration of the “off” period, and, if the off durationis long enough, the correction eventually causes the tuner to return tothe home position. For embodiments using a klystron, the RF outputfrequency is adjusted electronically.

THE FIGURES

The foregoing summary of the invention, as well as additional aspectsand features, will be better understood from the following detaileddescription, taken in conjunction with the appended Figures, in which:

FIG. 1 is a block diagram of a linac-based X-ray system in accordancewith an embodiment of the invention.

FIG. 2 illustrates in cross-section a linac in accordance with anembodiment of the invention.

FIG. 3 illustrates the relationship between RF pulses and electron beampulses in accordance with an embodiment of the invention, including theuse of beam pulse duration to achieve pulse-to-pulse dose control.

FIGS. 4A-4E are timing diagrams showing an interleaved sequence ofstable pulses in accordance with an embodiment of the invention,including dose control.

FIG. 5 is a block diagram of a dual mode gun driver in accordance withthe invention.

FIGS. 6A-6D are timing diagrams illustrating the cool down cycle whichresults from intermittent scanning operation, and the improvementresulting from the cool down compensation system and method of thepresent invention.

FIG. 7 illustrates in block diagram form an alternative embodiment ofthe invention which offers dose-to-dose pulse control.

FIG. 8A shows in timing diagram form the pulse shapes of FIG. 3, anddistinguishing higher and lower energy pulses with a “normal” higherdose.

FIG. 8B shows in timing diagram form the pulse shapes corresponding tothose of FIG. 8A, but in reduced dose mode for both higher energy pulsesand lower energy pulses.

FIG. 9A shows the reduced dose pulse that results from the waveforms ofFIG. 8B, in a pulse train of the form ABABAB.

FIG. 9B shows the reduced dose pulse that results from the waveforms ofFIG. 8B, but where the reduced dose pulses C are a single energydifferent from either A or B.

FIG. 10A shows a combined pulse train of a single energy level for thereduced dose portion and a single energy level for the higher doseportion.

FIG. 10B shows the transition time between the reduced dose mode and thenormal dose mode for the single energy system of FIG. 10A.

FIG. 11 shows a single grid gun driver in accordance with an aspect ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, an X-ray scanning system according to anembodiment of the invention is shown in block diagram form at 100. Insuch an embodiment, external power and signals 105 are received by acontrol processor 110. Included among the external signals are,typically, one or more trigger signals indicating that the user desiresto scan an object, for example a cargo container on a vehicle passingbefore the scanning system. The control processor 110 controls, directlyor indirectly, the operation of the remaining functional blocks shown inFIG. 1 by virtue of signals sent on internal bus 115, which, forsimplicity, is shown combined with internal power.

In response to the trigger signal(s), the control processor 110 sends,depending upon the implementation, a plurality of signals to initiategeneration of an X-ray pulse. In particular, the processor 105 sendscontrol signals to a high voltage power supply 120 and an associatedmodulator 125 which receives the output from the supply 120. The supply120 can be, for example, a Lambda LC1202. The output of the modulator125 supplies a high voltage output to a pulse transformer 130, typicallyimmersed in an insulating tank for purposes of electrical isolation. Anaspect of the modulator is that can vary the voltages from one pulse tothe next, and can operate at pulse durations of 2.5 μsec or less, topermit operation at 400 pulses per second. The modulator may incorporatea pulse-forming network or PFN. A heater power supply 135 is associatedwith the tank and supplies the magnetron 140 or other suitable RF powersource. The pulse transformer 130 supplies high energy pulses, forexample 30-50 kV at 100-110 amps, to a magnetron 140 or other suitableRF power source. One suitable magnetron is the e2V model MG5193, whichhas an output of 2.6 MW at the normal S band frequency of 2.998 GHz.Another is the MG7095, also from e2V. Still other similar magnetrons areavailable from NJRC. The specific magnetron frequency is controlled by amechanical tuner 145.

The magnetron 140 outputs an RF power pulse, indicated at 150, at thefrequency determined by the tuner 145. As explained in greater detailbelow, the pulses received by the magnetron can be of different,pre-selected voltage and currents, thus causing the magnetron to outputpulses of different, pre-selected RF powers, for example, pulses of 40kV and 45 kV at 100 amps and 110 amps, respectively. Because of thenature of the magnetron, the different powers of the RF pulses alsoaffect the frequency of the output pulse, again as explained in greaterdetail below. The RF power pulses pass through an arc detector 155, anisolator 160, and then to a linear particle accelerator (sometimes“linac” hereinafter) 165. Suitable isolators are available from FerriteIncorporated. Conventional S-band waveguide 157 is used betweenmagnetron and linac. Depending upon the requirements of the particularlinac, for example of the type shown in FIG. 2, the pulses received fromthe transformer 130 can be in the range of, for example, 35-50 kV. Thelinac 165, which typically has an effective Q in the range of 2000-4000,but in any event less than 5000, receives the RF pulse. The tuner 145 isadjusted so that the RF pulses from magnetron 140 are within theresonance bandwidth of the linac 150. The pulses from the magnetron are,in an embodiment, substantially in the range of 2.5 MW, or between 2.0MW and 3.0 MW.

In the embodiment of FIG. 1, the control processor 110 sends a controlpulse to the modulator 125, and it sends a synchronized control pulse toa dual mode electron gun driver block 170. The timing of these controlpulses may be individually optimized. The dual mode electron gun driverblock 170, explained in greater detail hereinafter in connection withFIG. 5, drives an electron gun 175, the cathode of which is within thevacuum envelope of the linac 165. The gun 175 can be a triode gundesign. In an embodiment, the pulses of beam current from electron gun175 launch electrons into the cavities of the linac. The cathode voltageis substantially in the range of −10 to −20 kV. In an embodiment it is−20 kV. By timing the electron pulses at the linac to optimallysynchronize with the electric field created by the RF pulses from themagnetron, as discussed in greater detail hereinafter, the electrons areaccelerated by the linac to a desired energy level, typically in therange of two to ten or more MeV with, for at least some embodiments, aseparation between the energy levels of approximately 1 MeV or morebetween sequential pulses in a rapidly pulsed ABABABAB pattern. Thepulses are directed toward a target 180, for example tungsten, which,when hit with the pulse of accelerated electrons, emits pulses ofX-rays. As discussed below in connection with FIGS. 3 and 4A-4E, the RFpulses are, in an embodiment, somewhat longer in duration than theelectron gun pulses, such that the RF pulses can be thought of ascreating an envelope within which the beam current pulses occur. Tocontrol dose, the duration of the beam current is selected by thecontrol system 105, or can be pre-set during manufacture.

Because the transmission characteristics of the linac and magnetron varywith temperature and other environmental factors, an AFC circuit 185detects forward and reflected power from the linac, using dualdirectional couplers 190, and in turn controls the tuner 145 to ensure acontinuing match between the linac and the magnetron. in a manner knownto those skilled in the art.

In another aspect of the invention, in some embodiments, the AFC circuit185 also includes cool-down compensation, discussed in greater detailhereinafter in connection with FIG. 6. Because X-ray scanning systemsoperate intermittently, and the duration of the “off” periods isessentially unpredictable, the cool down compensation of the presentinvention operates the tuner during periods when the scanner is off tomaintain a good match between the linac's resonance bandwidth and theoutput frequency of the magnetron, thus providing improved operationalstability.

In addition, ancillaries 195 connect to the arc detector 155 and an ionpump 200 that feeds the linac 165, both in a manner known to thoseskilled in the art. Finally, a cooling system 205 cools portions of thesystem in a manner known in the art, for example, the modulator, thepulse transformer tank, the linac, the target, and the isolator, asindicated by the dashed line 210. Suitable cooling systems are availablefrom OptiTemp, and can be chosen dependent upon temperature and coolingrequirements of the linac system.

With reference to FIG. 2, a linac 165 in accordance with an embodimentof the invention can be better understood. The linac 165 is configuredto provide X-ray pulses of different and controllable energies and dosesin an alternating fashion. The linac comprises an electron gun 175,which can comprise a gridded electron gun. The beamline 220 is comprisedof the main cylindrical section, and is comprised of cavities 225 thathave been designed in order to accelerate electrons from the cathode ofthe electron gun up to speeds that correspond to energies in the MeVrange. The cavities of the beam line can be inline coupled orside-coupled, and the cavities are also designed for resonance with theRF source, such as the magnetron 140 in FIG. 1. A water-cooled targetassembly 230 is attached to the linac beam line, and houses a target180, which is made substantially of tungsten, or other suitablematerial. In an embodiment, the linac is designed to accelerateelectrons up to 6 MeV in energy and can also accelerate electrons to 4MeV in energy on an alternating pulse, interleaved basis, for thepurpose of generating X-rays with similar maximum energies per pulse. Anion pump 200 is connected to the vacuum envelope of the linac in aconventional manner. An RF window 235 is attached to the linac beamline,for the purpose of input coupling of RF power. Between the RF window 235and the beam line 220 is a coupling iris 240 or opening, the size ofwhich can be adjusted in order to assist in tuning the effective Q ofthe linac assembly.

As discussed above, one aspect of at least some embodiments of thepresent invention is to generate interleaved sequences of X-ray pulsesof at least two differing energies, for example 6 MeV and 4 MeV. In anembodiment, and with reference to FIGS. 3 and 4, the interleavedsequence of pulses rapidly alternates between the higher energy and thelower energy pulses, e.g., ABABABABAB, although other embodiments canprovide different sequences of interleaved pulses, such as AABB or anyother desired sequence. For purposes of clarity only, the alternating,or ABABABAB, sequence will be described in greater detail hereinafter,as those skilled in the art will understand how to provide othersequences, given the teachings herein. It will also be appreciated thatmore than two energy levels of X-ray pulses can be generated with thecurrent invention as long as the frequency shifts of the magnetronpulses remain within resonance bandwidth of the linac and are properlymatched to the beam current. For purposes of illustration, use of thepresent invention to generate an AB sequence is explained below, withreference to FIG. 1 and the timing diagrams of FIG. 3.

To generate an alternating sequence of lower and higher energy X-raypulses, the control system 110 instructs the modulator 125 to supply adifferent level of power, to the pulse transformer 120 for eachdifferent energy level of pulse. This, in turn, varies the powersupplied to the magnetron 140, such that the alternating higher andlower RF pulses are supplied by the magnetron to the linac, as shown inFIG. 3 at 300 and 305. By varying the power to the magnetron, thefrequency of the output of the magnetron changes, for example byapproximately 10 kHz per amp, or 100 kHz when the current supplied tothe magnetron varies from 110 amps for 6 MeV to 100 amps for 4 MeV, asan example. As discussed above, the Q of the linac is configured toprovide a resonance bandwidth of sufficiently more than 100 KHz, so thatboth the higher and lower frequencies output by the magnetron remainwithin the operating range. Again as discussed above, in an embodiment,the Q of the linac is preferably in the range of 2000-4000, and a Q of3000 has been demonstrated to operate well. In any event, the Q is lessthan 5000.

As shown in FIG. 3, within the pulse duration of the magnetron pulses300 and 305, the electron gun driver also generates current pulses ofdifferent energies, as shown at 310 and 315. The A pulse is of a desiredelectron and X-ray energy and X-ray dose, and the B pulse is of adifferent electron and X-ray energy and X-ray dose that is at least 1 or1.5 MeV different from the energy associated with the A pulse. Themagnetron current pulses are delivered by the pulse forming network andmodulator of a preferred embodiment of the invention, and have aduration that is set by the design of the pulse forming network withinthe modulator 125 shown in FIG. 1. Other modulators could be used wherethe magnetron current pulse duration can be changed. In an embodiment,the duration of the current pulse is between 2 and 2.5 microseconds forboth A and B pulses, and the repetition rate of the pulses is on theorder of 100 to 400 pulses per second, or whatever is allowed by themaximum average power dissipation and delivery that a given magnetroncan tolerate. For a faster magnetron, a rate of 1,000 pps, and, at theother end of the range, a rate of 1 pps can also have utility in someapplications.

The higher current magnetron pulse is typically correlated to the higherenergy X-ray pulse, and the lower magnetron current pulse is typicallycorrelated to the lower energy X-ray pulse. The high and low currentscause a higher and lower RF peak power to be delivered to the linearaccelerator. The difference between these currents is limited in orderto limit the magnetron frequency shift that occurs as a result ofdifferent voltage and current applied to the magnetron, in order thatboth the A and B magnetron frequencies are both well matched to theresonance bandwidth of the linac while still obtaining at least 1 MeV ofdifference between the X-ray energy of the A pulse and that of the Bpulse, when pulsing in an interleaved sequence.

As discussed in connection with FIG. 5, below, the gun driver can becontrolled from the control system to alter amplitude, duration andtiming of the beam current pulses. The gun current magnitudes arecontrolled by the gun driver, so that different beam currents for the Apulse and the B pulse are controlled. The combination of different RFpower and different beam currents allows control of the energy of the Aand the B pulses, in accordance with the method of the presentinvention. In an embodiment, the energy difference between the A and Bpulses is 2 MeV, although in other embodiments the difference can rangefrom 1 to 5 MeV. Further, the gun current pulse durations of the Apulses and the B pulses are independently controlled by the gun driverin order to control the X-ray dose on a pulse-to-pulse basis. Theduration of the voltage pulse to the grid substantially controls theduration of the gun current pulse, and therefore the duration of theX-ray pulse. If all other parameters are fixed, and only the pulseduration parameter is adjusted, then the dose per pulse will be adjustedwhile the X-ray energy for a given pulse will remain the same. In anembodiment, the dose of the A pulse and the dose of the B pulse aresubstantially equal. In another embodiment, the ratio of the A dose andthe B dose is adjusted to a desired ratio other than one.

In addition to the adjustment of gun current pulse duration to controlthe dose per pulse, the timing of the start of the gun current pulsewith respect to the RF pulse can also be adjusted. In a preferredembodiment, the leading edge of the gun current pulse begins early inthe “flat” part of the RF waveform. In other embodiments, the leadingedge of the gun current pulse can be started later with respect to the“flat” part of the RF waveform.

The overall AB sequence can be appreciated from FIGS. 4A-4E, in whichFIG. 4A illustrates two sequences of X-ray pulses with an “off” periodof indeterminate duration in between. FIG. 4B illustrates the dose perX-ray pulse for the higher energy X-ray output, while FIG. 4Cillustrates dose per pulse for the lower energy X-ray output. Both FIGS.4B and 4C illustrate a range of dosage amounts with a variance whichrepresents acceptable stability, typically about 10 percent or less, or2% or less, or 1% or less. FIG. 4D illustrates the current supplied tothe magnetron for each of the lower and higher energy pulses, while FIG.4E illustrates the beam current supplied to the linac for the higher andlower energy pulses. It will be appreciated that the beam current islower for the higher energy pulses than for the lower energy pulses inan embodiment. In one embodiment of the invention, the peak beam currentis controlled to between approximately 25 mA and 125 mA when high energypulses are desired, and the peak beam current is controlled to betweenapproximately 125 mA and 250 mA as associated with the low energypulses. Other beam currents are selectable with the linac system of theinvention. In an embodiment, the beam currents can be precisely tuned inconjunction with the RF power in order to precisely provide a desiredX-ray energy, and thus the X-ray energy can be alternated on a rapidpulse to pulse basis in an ABABABAB interleaved manner.

As discussed above, the pulses generated by the electron gun driver areadjustable in amplitude, duration and timing, which permits the beamcurrent pulses to be synchronized and matched with the RF pulses togenerate X-ray pulses of different, controllable energy levels, withconsistent, controllable dose per pulse. FIG. 5 illustrates anembodiment of a dual mode gun driver suitable for generating pulses of2.5 microseconds or less, suitable for cargo scanning functions whichtypically operate from 100 to 400 pulses per second, in accordance withan embodiment of the invention. Higher repetition rates are possible ifstandard electrical and thermal parameters are considered, as will beappreciated by those of skill in the art.

The gun driver 500 shown in FIG. 5 essentially comprises twoindependently controllable driver modules, one for the high energypulses, indicated at 505, and another for low energy pulses shown at510, where each module connects to the grid of the gun 175 (FIG. 1) atthe appropriate times through control of high voltage switchingtransistors, and supplies the appropriate pulse at appropriate butdifferent times as shown in FIG. 4E. The high energy module 505comprises a heater supply 515 and high energy grid top power supply 520,with the output of the grid top supply 520 connected to the grid outputthrough a switching transistor indicated at 525. The low energy module510 comprises a low energy grid top power supply 530 and a grid biassupply 535, and the low energy grid top power supply connects to thegrid via a switching transistor 540 and a diode 545, while the grid biassupply connects to the grid through a switching transistor 550. The gundriver also comprises a high voltage filter module 555 and a highvoltage power supply 560 in a conventional manner.

The performance of the gun driver modules, including the high voltageswitching transistors, is controlled from the control system 110 and bus115, as more generally shown in FIG. 1. Feedback signals from eachmodule are also used, as is common in the art of electronics. The gridmodulating modules 505 and 510 are referenced to the cathode voltage, asshown, and apply voltage to the grid of a triode gun that is part of thelinac, shown at 175 in FIG. 1. As is known in the art, the control ofgrid voltage is a common method for controlling the gun current emittedfrom the cathode of a triode or gridded electron gun. In the preferredembodiment, one grid modulator is used to control the grid voltageassociated with the A pulse, and the other grid modulator is used tocontrol the grid voltage associated with the B pulse. In anotherembodiment, a single grid modulator can be used if the grid voltage canbe changed accurately to desired values on a pulse-to-pulse time basis.The grid voltages are controlled in order to control the gun currentthat is launched into the linac, as the magnitude of the beam current inthe linac is a function of the gun current. The independent control ofthe amplitude of the beam current, combined with the independent controlof the RF power, is used to control the X-ray energy on a pulse to pulsebasis.

The duration of the grid pulse is used to control the dose of a givenX-ray pulse. As discussed above, a shorter pulse yields a lower dose perpulse, whereas a longer pulse corresponds to a larger dose per pulse.

Referring next to FIGS. 6A-6D, a different aspect of the presentinvention can be appreciated, by which pulses of consistent energy areprovided even with a “cold start” or after the linac system in thescanner has been off for an indefinite period. More specifically, FIGS.6A-6D depict both the effect of classic AFC techniques as well as thenovel cool down compensation technique of the present invention thatprovides stability from the first pulse in an intermittent pulsingapplication. Since the invention outlined above uses standard prior artAFC techniques, the interleaved ABABABABAB etc. output can be stableover long time periods, for example, seconds, and minutes. Longerdurations are possible as well. Prior art AFC techniques require atleast one feedback signal proportional to the reflected RF power fromthe linac to insure proper mechanical tuning of the magnetron, as in thecase of a preferred embodiment, and as is well known. In the case of aklystron, tuning is accomplished via an input RF driver, but the AFCstill requires feedback representative of the output of the RF sourcewith respect to the linac resonance. Since feedback techniques are used,many RF pulses (and therefore X-ray pulses in most systems) are requiredbefore the AFC subsystem has sampled enough information and adjusted themagnetron mechanical sufficiently that optimal and stable X-ray pulsesare generated. A conventional AFC system alone is not sufficient toallow highly stable operation from the first X-ray pulse after an “off”period without RF feedback. In an embodiment of the invention, in orderto take the same linac system with AFC and provide optimal and stablepulsing from t=0, or what is sometimes called a cold start, a cool downcompensation (“CC”) algorithm has been invented and is used.

With classic AFC circuits, the AFC compensates for thermal effects orlong term drift effects that cause a drift in the X-ray pulseperformance. If a drift or deviation in performance takes place overmany pulses, and if it is due to a correctable frequency mismatchbetween the RF source and the linac resonance, then the classic feedbacktechniques used in prior art AFC subsystems can be used to stabilize thesystem. However, AFC designs and methods typically require many pulsesto correct the position of the magnetron tuner and so, if only the AFCis used and if the pulsing is intermittent, that is, shut off for anirregular period of time and then turned back on, the dose and/or energyper X-ray pulse for some number of initial pulses can deviatesignificantly from the intended dose or energy per pulse.

FIG. 6A depicts the operation of the magnetron in the context of aclassic heating and cooling curve as shown in FIG. 6B. The shape of thatcurve is generally associated with the deposition of heat duringpulsing, and then the dissipation of heat once pulsing stops. This is aclassic phenomenon known to those of skill in the art, and when heatingis occurring due to pulsing, a classic AFC can help maintain frequencymatch between a magnetron and a linac. However, when pulsing is turnedoff, then the system will cool, but the AFC cannot adjust the tuner inthe same manner because it receives no feedback signals. Theconventional feedback signals like forward RF power and reflected RFpower do not exist for the AFC during “off” or non-pulsing periods. Whena feedback signal is removed, prior art AFC subsystems have no inputinformation with which to drive a motor or tuner, and may maintain themotor and tuner position that was appropriate for the most recent pulse,or some other position, but that is not appropriate for the next pulseat some indeterminate future point in time. This can result innon-optimum X-ray output or output at unintended values once the systemis restarted, because the characteristics of the linac have changedduring the off period.

In an embodiment of the invention, cool down compensation logic isprovided in the AFC circuit (185 in FIG. 1) that causes the position ofthe tuner to be changed during non-pulsing periods without the need forfeedback, so that the output frequency of the magnetron remains properlymatched to the linac resonance characteristics despite the cooling thatoccurs during an “off” period. In an embodiment, this technique canachieve stabilities significantly better than 10%, and in some instancesapproximately 1%. The cool down compensation logic can be a look uptable (LUT), an algorithm, or other implementation, and is addressed bythe AFC circuit when no pulses are detected for a predetermined periodof time, for example, one second.

FIG. 6C is a timing diagram that shows the movement of the tuner duringcooling periods shows the changing tuner position that follows, andcompensates for, the cooling that occurs when scanning is not active.FIG. 6D is a plot of dose per pulse, and illustrates that, as the resultof the cool down compensation, the first pulses after a restart aresubstantially identical in energy and dose as the pulses that occurredduring prolonged scanning with the AFC circuit active. In an embodiment,the AFC circuit with cool down compensation logic drives a steppermotor, and the stepper motor moves the tuner on the magnetron in aconventional manner.

The cool down compensation data used to populate a LUT or define analgorithm, as discussed above, can be developed as follows. For a givenRF value, a proper magnetron starting or “home” tuner position isdetermined by very low repetition rate operation of the linac system, atthe desired energy and dose per pulse. For example, this home tunerposition can be determined at 1 to 10 Hz operation, and can be set by anoperator who optimizes system performance vs tuner position, eithermanually or with a classic AFC. The very low repetition rate simulates ascenario where very little heating is occurring. This home tunerposition is recorded in system memory. This type of operation simulatesthe behavior of a linac system with very little stored heat in eitherthe linac or magnetron. In a preferred embodiment, the magnetron tuneris driven by a stepper motor, and the number of steps away from a motorreference point or mechanical stop is used as a proxy for tunerposition. As an example, the home position may be determined tocorrespond to 50 steps away from the reference point or mechanical stop.A tuner position associated with full power and full repetition rateoperation may achieved by 100 or 150 steps away from the reference pointor mechanical stop, as an example

For the CC technique of the present invention to cause optimal tuningduring non-pulsing, a calculation or look-up table is generated withmotor positions that correspond to substantial matching betweenmagnetron frequency and linac frequency during the time intervals whereno pulsing is commanded. After very long off periods, like minutes, theproper motor position corresponds very nearly to the “home” position.After long “on” periods, for example after minutes of operation, themotor position determined by a classic AFC will be many steps away fromthe home position, in order to tune the magnetron to the linac resonancein a warmed state. For “off” periods of varying durations, the optimalmotor position will be somewhere between these two positions, and mustbe determined.

A variety of techniques can be used to determine optimal motor and tunerposition in the absence of pulsing. For an embodiment of the system, anoutline of one procedure that can be used to develop the tuner positionsduring cool down, and thus the entries for a LUT or other correctiontechnique, is as follows:

1. The system is operated at full energy and power, so for example, themagnetron output is 2.6 MW, and it is pulsed at 400 pps with pulsedurations of about 2 microseconds for the flat-top of the RF pulse. Thismode corresponds to the maximum heat deposition to the system in anembodiment. A standard AFC circuit maintains the match between themagnetron frequency and the linac resonance during this pulsing period,with a time constant on the order of 0.5 seconds. Near steady state canbe achieved in times on the order of a few minutes for this approach.

2. The system is turned down to 10 pps abruptly, and the AFC is allowedto tune the magnetron tuner in a standard way, typically by driving astepper motor. Every 5 seconds, an operator or a data acquisition systemrecords the position of the stepper motor, over a period of severalminutes.

3. A plot of stepper motor position is created, which shows steps on theY axis, and time on the horizontal axis.

4. The plot in step 3 above can be fit to an exponential decay with atime constant, or it can be used to create a look-up table. Thecalculated exponential decay can be used to calculate the proper tunerposition as a function of its most recent position; in a preferredembodiment, the farther a tuner has been stepped from its zero heat or“home” position, the larger its correction per unit time towards thezero heat or “home” position should be.

In an embodiment, a rate of change in terms of steps per second towardshome may be calculated or measured from the data collected in steps 1 to3 above. The rate of change can be plotted as a function of the stepsaway from home. In a preferred embodiment, the optimal rate of change islargest for position changes that are large with respect to the homeposition. With the CC active, for any position of the motor that drivesthe magnetron tuner, there is an associated rate-of-change insteps/second corresponding to that position. This rate of change caneither be calculated by determining how many steps the AFC moved betweenintervals in the procedure 1 to 3, or it can be derived from a fit tothe exponential curve to the position vs time data collected in steps 1to 3. Alternatively, a look-up table can be created.

In an embodiment, an optimal expression of the steps per second as afunction of steps from home can take the form

Y=a*e ^((b*x))

where Y is the optimal steps per second, x is the steps from home, and aand b are constants determined by a mathematical fit to experimentaldata for the linac system in an embodiment. In general, a is acoefficient representing the magnitude of the exponential equation, andb is a coefficient for curve-fitting. For example, in an embodimentwhere the stepper motor has on the order of 100 steps of range, a can be0.13, while b can be 0.034. Those skilled in the art will recognize thatthese coefficients can be different numbers depending on the thermalbehavior of the system, or the behavior of the system in response toother parameters. When the CC is used to move the stepper motor thatmoves the magnetron tuner, and the CC uses the formula, the pulsingperformance in an intermittent pulsing scenario was very nearly optimalfor every pulse, as shown in FIGS. 6D and 6E.

In an alternative embodiment, the following procedure can be used:

1. Warm up at 400 pps for 3 minutes, note warmed-up tuner motor stepposition with respect to home (position as controlled by classic AFC).

2. Shut off for 5 seconds, turn back on, record tuner motor stepposition after 2 seconds (position as controlled by classic AFC).

3. Warm up again at 400 pps to same warmed-up tuner motor step positionas #1.

4. Shut off for 10 seconds, turn back on, record tuner motor stepposition after 2 seconds (position as controlled by classic AFC).

5. Warm up again at 400 pps to same warmed-up tuner motor step positionas #1.

6. Shut off for 15 seconds, turn back on, record tuner motor stepposition after 2 seconds (position as controlled by classic AFC).

Continue this sequence for up to 2 minutes of shut off time. Plot thedata as tuner motor position in “steps-from-home” (y axis) vs “cool-downtime” (x axis).

7. Fit an exponential to that data.

8. Take the derivative of that data, and plot it on the same graph,which will now depict both the exponential fits to “steps-from-home” and“steps/second” vs “cool-down time”.

9. Create a new plot of “steps/second” vs the “steps-from-home”.

In an embodiment, this table created from such techniques is used by theAFC with CC during customer-triggered intermittent operation to providethe rate at which the tuner motor should be moved any time the RF isturned off, based upon the last position where the tuner motor was leftby the AFC. In this embodiment, the AFC controls the tuner motorposition while RF and X-rays are pulsing, and the CC controls the tunermotor position when RF and X-rays are turned off.

In some cases, further improvement resulting in better pulse-to-pulsestability is desirable. In the application of cargo scanning this canfacilitate better imaging, and in the application of radiation oncologythis can facilitate more stable dose delivery. For example, an RMSstability of the X-ray dose per pulse of nearly 1%, or less than 1%, isdesirable. In the case of the linac operating in dual energy on analternating basis, the dose per pulse at one energy may be less stablethan the dose per pulse at a different energy. To enhance stability ofthe dose beyond what is achieved by AFC alone, additional techniques canbe used. One technique, which uses substantially the hardware structureshown in FIG. 1, takes a digital output from the AFC circuit 185 duringX-ray pulsing bouts in order to generate a correction signal for theprogramming voltage for the modulator 125 that drives the magnetron 140.The programmed voltage for steady state magnetron operation is typicallya fixed value, for example, a high voltage power supply programmedvoltage of 20 kV will result in a PFN output of approximately 10 kVwhich results in a pulse to the magnetron of 45 kV (after a 4.5:1 pulsetransformer.) This results in an RF output of approximately 3 MW, whichin combination with a specific gun current will result in a specificdesired energy, such as 6 MeV. Using the AFC circuit's digital output ofthe AFC stepper motor position, a correction value to the normally fixedprogrammed voltage can be created, using the reported AFC stepper motorposition with respect to “home” or zero position. In a preferredembodiment, the correction value can be positive or negative, and isadded to the steady state programmed voltage that corresponds to one ofthe desired pulse energies. In a preferred embodiment, this correctionvalue contains a term proportional to the AFC stepper motor positionwith respect to the “home” position that corresponds to tuner positionwith no system heating, and this correction value also contains a fixedoffset term. Either term of the correction value can be positive ornegative, depending on the direction of desired correction to thepulse-to-pulse X-ray dose over a typical 30 to 60 second scan. Thecorrection value can be added to the programmed voltage to just one ofthe alternating energy pulses (either the high or the low energy), or toboth, or a differently scaled correction value can be generated for eachof the energy settings. By adjustment of the correction value, enhancedX-ray stability over the duration of a typical scan is achieved,resulting in an improved image in a security scanning system.

Another method to stabilize the X-ray dose per pulse is by measuringtarget current on a pulse-to-pulse basis. One approach for accomplishingthis is where the target assembly has been electrically isolated fromthe linac body, and current due to the electron beam striking the targetis measured with a simple circuit. Peak target currents of between 25 mAand 250 mA during a pulse are typical values, which allows generation ofa measureable signal voltage across a resistor in series with the targetcurrent. If the electron energy is constant, the dose for a given X-raypulse or series of pulses is directly proportional to target current.Therefore, if electron energy is constant, a feedback loop can beimplemented using target current as the measured parameter that is aproxy for X-ray dose. The feedback loop stabilizes target current (andthus X-ray dose), on a near pulse-to-pulse basis by adjusting the highvoltage power supply program voltage (the voltage that drives themagnetron, as discussed above). Another method to achieve this effect isto measure the integrated target current over an individual pulse, inorder to calculate the total charge delivered in an individual pulse. Inthis alternate method, using integrated target current per pulse, aservo loop that adjusts pulse duration at the target is used tostabilize the integrated target current per pulse with respect to adesired value. In a system with a triode or gridded gun and anindependent triode gun driver, a straightforward way to control theduration of a pulse at the target is by controlling the duration of apulse to the grid as delivered from the gun driver. Certain solid statemodulators may also facilitate this form of stabilization. However, thisapproach can suffer from certain instabilities if a change in energyoccurs.

A still further alternative, shown in FIG. 7, offers some advantagesbecause it measures actual dose on a pulse-to-pulse basis rather thanthe integration-based averaging performed by many other designs, and, bymeans of a feedback loop, uses the measured dose to stabilize thesuccessive generated doses. The feedback loop can, for example, controleither the voltage level at which the magnetron is driven, which adjuststhe RF power deliver to the linac beam line; or, alternatively, thepulse duration of the gun driver that drives an electron gun with acontrolling grid, where longer pulses correlate to a greater dose perpulse and shorter pulses correlate to lower dose per pulse.

For the purpose of measuring a signal that is proportional to dose,either a scintillator crystal based detector or an ion chamber can beused. A challenge to this approach is the development of a detector withan appropriate response to the X-ray dose, and ideally with a signallevel and speed that allows a pulse to pulse measurement of the X raysignal. Scintillator crystals and diode detectors are used for thispurpose in the detector arrays used for X-ray imaging of cargo—they aredesigned for detection of the relatively lower level signals that passthrough the cargo. A detector using a cadmium tungstate crystal andphotodiode can be used, as is well known in the detection of gamma levelX-rays. Care must be taken to keep X-ray levels low enough to minimizedamage to the crystals, but high enough to provide an adequate signalfor the photodiode detectors to measure with respect to electrical noiselevels, as is known in the art. Detectors are available by custom orderfrom Berkeley Nucleonics Corporation, of San Rafael Calif. A challengewith an ion chamber is the development of a chamber with a sufficientlyfast response time and sufficiently high signal levels. One such ionchamber is the A12, which can be procured from Standard Imaging inMiddleton Wis.

The signal from either of these properly designed dose detectors can beused to measure dose per X-ray pulse, and this signal can be used inconjunction with a feedback circuit or computational technique for thepurpose of maintaining an X-ray dose per pulse that is more stable on apulse to pulse basis than may otherwise be achieved. The magnetronvoltage is adjusted with a correction depending upon an error signalgenerated by the difference between the actual measured dose-per-pulse,and the desired dose-per-pulse.

Still referring to FIG. 7, an embodiment of the invention using apulse-to-pulse dose detector can be better appreciated. It will beapparent that the basic structure is that of FIG. 1, and like elementsare indicated with like numerals. As shown in FIG. 7, a pulse-to-pulsedose detector 105, typically either scintillator-based or ionchamber-based, detects a portion of the X-rays emitted from the target,and generates a feedback signal 107 which provides an additional inputto the control system 110. The scintillator-based detector is preferredin at least some embodiments. An ion chamber can be designed tointercept and measure most of the X-ray beam by placing it close to thetarget. However, the ion chamber measures dose or signal in a differentway than the scintillator crystal and diode combinations used in thearrays used for cargo scanning. The ion chamber measures a signalproportional to the amount of ionized gas between two conductors, butthe scintillator uses a crystal that emits visible light with arelationship to the X-rays incident upon the crystal, with a photodiodeand electronics being used to measure the light output of thescintillator crystal. Therefore, it is preferable in some embodiments tostabilize pulse-to-pulse dose from the linac system using a scintillatorbased detector.

In a typical design, a collimator (not shown) is used to generate thedesired X-ray pattern for cargo scanning. In such cargo scanners, theX-ray pattern is commonly a fan beam having an included angle between 50and 90 degrees, and a width of 2 mm to 3 mm. This line beam of MeVX-rays is used to illuminate the detector array in a scanning system,with this array made of scintillator crystals and diodes and necessarysignal processing electronics. In the presently described embodiment ofthe invention, the pulse-to-pulse detector 105 is placed either withinthe linac system cabinetry itself, or near it, in a position that isnever obscured by the cargo to be scanned. The detector can be placed atthe edge of the X-ray fan beam, or just outside of that fan beam, inorder to measure a signal that is directly proportional to thatdelivered in the fan beam for imaging purposes. Other locations are alsopossible, such as behind the electron gun, as some X-rays are emittedfrom the target in directions other than the fan beam direction. In anembodiment, the detector is placed in a location that providessufficient signal for the scintillator-diode detector combination, andprovides a signal that is sufficiently proportional to the dose perpulse delivered into the fan beam used for imaging.

Use of an ion chamber for measuring dose in a linac system is well knownin the art. However, it is novel to use of a detector that is the sameas that used in the imaging array, but here for the purpose of measuringand stabilizing the X-ray dose on a pulse-to-pulse basis. Thepulse-to-pulse dose detector signals are measured by the control system,where the measured signal associated with the dose per pulse is comparedagainst a reference value. Standard feedback techniques can be used togenerate a control signal to the appropriate linac subsystem for thepurpose of stabilizing the dose per pulse. In a preferred embodiment,the voltage level of the HVPS (120) is adjusted on a pulse-to-pulsebasis in order to provide an improved stability per pulse. In anotherembodiment, the duration of the gun pulses is controlled in order toadjust the dose per pulse and improve the stability per pulse; in thisembodiment, the duration of the pulses delivered to the grid of theelectron gun is adjusted in order to adjust the dose per pulse. In athird embodiment, if a modulator is used to drive the magnetron thatallows adjustment of the duration of the pulse duration to themagnetron, such as a modulator available from Scandinova Electronics ore2V, then the duration of this pulse to the magnetron can be used toadjust the dose per pulse. In all cases, benefit is provided by using apulse-to-pulse dose detector that measures the dose per pulse in amanner that is very similar to, or exactly the same as, the manner usedby the imaging array.

As discussed above in connection with FIG. 5, an embodiment of the linacsystem of the present invention comprises a dual grid gun driver for thepurpose of providing alternating pulses of potentially differentvoltages and durations to the grid of the triode gun of the linac. Thegun driver is used in conjunction with an electron gun in order tolaunch current pulses into the RF-excited linac, where the electronpulses are accelerated, and strike into a target in order to producedesired X-rays. The dual gun driver of FIG. 5 provides completelyindependent control of the amplitude and duration of an A pulse trainand a B pulse train in a sequence of pulses that can be ABABABAB, etc.,where A and B are used to represent pulses of different characteristicsIn addition, it will also be appreciated, from the teachings herein,that the pulse train generated by the present invention can beconfigured as AAAABBBBABABABA or any arbitrary pattern that is desiredfor a given application.

The linac system is operated to provide controlled, accelerated electronpulses in this manner, in order to provide controlled X-ray pulses withcharacteristics that are desired for cargo scanning and securitysystems, as well as medical systems. These X-ray pulses are typically inthe MeV energy range, from approximately 0.5 MeV up to 15 MeV. Asdiscussed hereinabove, in an embodiment, the A and B pulses are at 6 MeVand 4 MeV in energy, with independently controllable doses, but can beother energies as well. In addition, in an embodiment, patterns ofpulses such as ABCDABCD, where C and D represent pulses having stilldifferent characteristics from A and B, or other patterns of pulses ofcontrollable energy and controllable dose can also be created by thepresent invention.

In some embodiments, if alternating amplitudes of current are notrequired, the two grid driving modules shown in FIG. 5 can be reduced toone module, even if alternating or otherwise controllable pulsedurations are required in a scanning application. The two grid drivingmodules of FIG. 5 can also be reduced to one module while still allowingalternating grid voltages if the module includes a rapidly adjustablegrid top power supply. That is, if the grid top supply is designed in amanner to allow control of the grid voltage at a speed that is fasterthan the time between desired X-ray pulses, then the same ABABABABfunctionality can be achieved, or AAAABBBBABABABA or any arbitrarypattern that is required. For example, an arbitrary pattern such asABCDE may be required in an application, where A, B, C, D, E representpulses of more than two different energies, and, in the instant case,five different energies. The resonant bandwidth of the linac imposes anupper limit on the range of different energy pulses. Other architecturesare also possible, that allow grid voltage control and gun pulseduration control. In a preferred embodiment, the pulsed voltage appliedto the grid of the gun is controlled between −80V and +300V with respectto the cathode voltage, and the cathode voltage is held to a voltagebetween −5000V and −20,000V with respect to ground. In a preferredembodiment, the durations of the grid pulses applied to the grid of thegun of the linac are controlled from between 50 nsec and 5 microseconds.These ranges are typical but do not limit the scope of the invention.Voltages such as these are used to provide peak currents at the targetof as high as 200 mA, but can be controlled to as low as 1 mA or lesswhen very low dose operation is required.

Duration control can range from a “full duration” that approaches theduration of the RF pulse (2 microseconds to 5 microseconds in preferredembodiments), down to a “minimum duration” which would provide theminimal useful integrated pulse current (such as below 100 nsec in apreferred embodiment). Such a range of control can allow rapidpulse-to-pulse adjustment of X-ray dose per pulse over a range of 5 to1, or 10 to 1, or 20 to 1, or 50 to 1, or 100 to 1, or 200 to 1,depending on the desired effect on the X-ray pulsing.

For cargo and security scanning applications, it is desirable in someembodiments to scan a portion of the vehicle using a different dose thanfor other parts of the vehicle. Thus, for example, in somecircumstances, it is desirable to reduce the X-ray dose during a scan ofa cargo container or vehicle at one or more selected points. One reasonfor this is to reduce the amount of radiation required in any givenscan, thereby reducing the amount of radiation to which surroundingpersonnel and operators will be exposed during a scan or during acollection of scans. Another reason for this is to scan part of avehicle that may still have a driver and/or occupant or occupants inplace, in which case sufficiently low dose rates are required for safetyof the driver or other occupants. The linac system of the presentinvention, with either a dual grid gun driver or single grid gun driver(such as FIG. 11, discussed below), is controllable in a manner thatenables this type of operation.

In a case where alternating energy is required, but a portion of thescan may need to be taken at a much lower dose per pulse, the pulsepattern may be abababababababaABABABABABABABA, or another arbitrarypattern, where the upper case A and B illustratively represent pulsesare of alternating high and low energy, and of relatively higher dose,and the lower case “a” and “b” pulses are of the same or similaralternating high and low energy, but of relatively lower dose. FIGS.8A-8B show the configuration of magnetron pulses 800 and 805 and gundriver pulses 810 and 815 appropriate for generating such pulses ofdifferent dose, where FIG. 8A replicates FIG. 3 as representative of afirst, or “normal” dose, while FIG. 8B illustrates the different pulsecharacteristics that yield a second, lower, dose per pulse. The factorin doses may be of 5 to 1, or 10 to 1, or 20 to 1, or 50 to 1, or 100 to1, or 200:1 or even more, depending on the object being scanned. Forexample, while the dose delivered by a ‘normal’ X-ray pulse for securityscanning may be about one Gray or lower, some security scanning requiresscanning the cargo area of a truck with a dose as large as 10 gray/min.In contrast, a driver sitting in the cab of that truck should receive nomore than about 0.25 microsieverts, where the conversion ratio is 1 Gray(Gy)=1000 mSieverts (mSv). For humans, the annual safe limit forradiation exposure is generally accepted as about 5000 mSv, Calculatingthe exact ratio is difficult because distance from the source to thetarget and time of exposure are also factors, but it can readily beappreciated that the low dose pulse is typically significantly less thana normal dose pulse.

The change in dose can be accomplished by a change in gun pulse durationfrom one pulse to the next, or from “a” and “b” to “A” and “B”. Afurther change in dose can be accomplished by changing the grid voltage,which is known by those of skill in the art to change the amplitude ofthe current pulse that is launched from the triode electron gun into thelinac. Changing only the gun pulse duration is a preferred method whenthe scanning application provides a benefit in that the acceleratedelectron energy that is associated with the “a,b” pulses and “A,B”pulses is preserved, which in an embodiment can be 6 MeV and 4 MeV. Anexample of the resulting pulse train is shown in FIG. 9A where thepattern of pulses shown at 900 is the “normal” dose, and the reduceddose mode is shown by the pulses at 905. In a preferred embodiment, thedose rates at 100 pps to 400 pps during “normal dose” scanning might bebetween 0.1 Gy/m in and 10 Gy/min, while the “reduced dose” section of agiven scan might be performed at dose rates that are 5 x, or 10 x, or 20x, or 50 x, or 100× less than the “high dose” scanning. Changing theamplitude of the gun pulse current can result in a change in the energyof the accelerated electrons in many linacs, due the effect of beamloading, which has been described. In the case of a reduced dose modewhere the electron current may be 10 mA instead of 100 mA associatedwith a 6 MeV “normal dose” pulse, the energy associated with a lowercurrent reduced dose pulse may be somewhat higher, such as 6.5 MeV,which is still acceptable and in some cases beneficial.

Those skilled in the art will recognize that the system of the presentinvention, using either a triode or a suitably fast gridded gun driver,can rapidly enable a series of reduced dose pulses immediately before,after, or during a series of normal dose pulses. Further, this techniquecan be advantageously applied to any linac system in a securityapplication, so long as the linac system uses a linac device thatincludes a triode or suitably fast gridded gun together with a gundriver as described herein. As discussed herein, the triode or griddedgun driver can be configured to contain either two grid modulators orone grid modulator.

Another desirable pattern may be cccccccABABABABABA, as shown in FIG.9B, where the series of “c” pulses 910 are of a single controllableenergy and controllable dose, and the series of “A” and “B” pulses 900is a series of pulses of alternating controllable energy andcontrollable dose. In a preferred embodiment, the dose of the “c” pulsesis at a rate that is 5 x, or 10 x, or 20 x, or 50 x, or 100 x or 200×less than the “high dose” A, B scanning. The energy of the “c” pulsescould be the same as either “A” or “B”, or may be selected to bedifferent.

Still another desirable pattern may be aaaaaaaaAAAAAAAA, where a pulsesof a single energy are used to scan, or ccccccAAAAAA, where the “c”pulses of reduced dose are a different energy from the “A” pulses. FIG.10A illustrates such a pattern of pulses, where 1000 indicates the “A”pulses and 1005 represents the “c” pulses. Either a dual grid or singlegrid gun driver can be used to control the linac in this manner. Thedual grid can facilitate an “a” mode that differs from the “A” mode inboth amplitude and in duration, whereas a single grid gun driver may beselected if only duration is the be changed on a pulse-to-pulse basis.If the “a” mode is to be the “reduced dose” pulsing mode, it can haveshorter duration with respect to an “A” pulse, lower current amplitudewith respect to an “A” pulse, or both. In a preferred embodiment, thelinac is operated at 6 MeV, or at 4 MeV in this non-alternating mode. Inanother preferred embodiment, the linac is designed to provide pulses ofan energy between 2 MeV and 3.5 MeV, with the amplitudes and durationsof the current pulses used to control the X-ray dose on a pulse-to-pulsebasis.

In some cases, the speed of a change to or from a “reduced dose” modewith respect to a “normal dose” mode does not need to be on a true pulseto pulse basis, but rather can take place over several pulses, as shownin FIG. 10B, at 1010. For example, a transition to or from a “reduceddose” from/to a “normal dose” modes may still be useful if it takesplace over 2 pulses or 10 pulses. In some embodiments of a gun driver,the design of the grid top power supply includes a stabilized DC powersupply, and a transition between voltages may take several milliseconds,which is still a usefully fast time scale.

In still other cases, it may be desirable to change the energy as wellin a “reduced dose” mode. In a preferred embodiment, the RF power can becontrolled from values used in “normal dose” mode to lower values thatcorrespond to a “reduced dose” mode that is both lower dose and lowerenergy. In a preferred embodiment, a capacitor charging high voltagepower supply is used to charge a line type modulator to a voltage valuethat can be controlled to different values on a pulse to pulse basis;the subsequent voltage pulse that is applied to the RF source cantherefore be controlled on a pulse to pulse basis. In a preferredembodiment, the RF source is a magnetron. In an embodiment, the gundriver parameters are controlled synchronously with the RF parameters toprovide the “reduced dose” mode.

Another method of reducing overall X-ray dose emitted by a system is thereduction of pulse rate. Thus, as is known in the art; a system operatedat 200 pps will provide one-half the output dose of a system operated at400 pps, if all other parameters are held constant. This lowerrepetition rate method can be used, but at some sacrifice to theresolution or speed of a given scan.

The triode gun driver, used to drive the triode electron gun of thelinac, can also be configured with a single Grid Modulator 1100, asshown in FIG. 11. The other elements can be substantially similar, suchas the High Voltage Power Supply 560 that is used to apply a DC highvoltage to the electron gun cathode, an associated Filter module 555.The DC high voltage can be approximately −20 kV in a preferredembodiment, but a range of −5 kV to −35 kV are often used with electronlinacs in the MeV energy range.

The Grid Modulator is used to pulse the grid of the triode electron gun.A configuration with only one Grid Modulator can be a preferred choicefor a linac that is used in primarily a single energy mode, i.e., doesnot require alternating back and forth between energies on a pulse topulse basis. However, the single grid gun driver and triode gun linac,such as the embodiment described in FIG. 11, still allows the dosegenerated by the linac to be controlled over a wide range on a pulse topulse basis, and still allows a rapidly controllable Reduced Dose Modethat is extremely useful in applications such as cargo scanning andsecurity scanning.

In an embodiment, the Grid Modulator 1100 includes a Bias Power Supply1125 that is used to provide a voltage to the grid with respect to thecathode voltage for the purpose of biasing the gun in an “off”condition—the application of bias voltage is used to prevents the gunfrom emitting current into the linac when it is not desired.

The Grid Modulator also includes a Grid Top Power supply 1105 that isused to set a voltage that will be pulsed to the electron gun grid whenpulses are desired. In a preferred embodiment, this voltage may beadjustable between −100V and +300V, and is matched to the requirementsof a particular electron gun. The voltage from the Grid Top Power supply1105 can be adjustable by the system controls, in order to control theamount of current in a pulse launched from the cathode of the triode guninto the linac structure where the electrons are subsequentlyaccelerated. The pulse of electrons is launched via switching signalsfrom the control system 115, which control Switch Drive 1110. The SwitchDrive 1110 in turn controls switches 1115 and 1120 to apply to the grideither the voltage from the Grid Top Power Supply 1105 or the “off” biasvoltage from the Bias Power Supply 1125. The amplitude of the currentpulse, the duration of the current pulse, and the timing of the currentpulse are all used to control the dose from the linac on a rapid basis,and in particular can provide a Reduced Dose Mode that is very useful.The Reduced Dose Mode is useful in both single energy linac systems andalternating energy linac systems.

In another embodiment, a single Grid Modulator gun driver is used inlinacs that require alternating energy or controllable energy on a pulseto pulse basis, by configuring the Grid Top Power Supply to be a fastresponding power supply instead of a DC power supply. A fast-respondingGrid Top Power supply facilitates rapid change of gun pulse amplitude ona pulse-to-pulse basis, and can be used in conjunction with RF controlin order to create pulse trains of varying energy, such as ABABAB, whereA is 6 MeV and B is 4 MeV. Other energy configurations are alsopossible, such as ABCABC, or other desirable combinations between 0.5MeV and 6 MeV, or 9 MeV or 10 MeV, as examples.

From the foregoing, those skilled in the art will recognize that a newand novel linac-based X-ray scanning system has been disclosed, offeringsignificant improvement in pulse-to-pulse stability for interleavedpulses of different X-ray energies, including pulse-to-pulse dosecontrol, all on a rapidly pulsed basis. In addition, in another aspectof the invention, cool down compensation permits substantially improvedstability in the dose and energy of the initial pulses after a coldstart, or after a restart after an off period of indeterminate duration.Given the teachings herein, those skilled in the art will recognizenumerous alternatives and equivalents that do not vary from theinvention, and therefore the present invention is not to be limited bythe foregoing description, but only by the appended claims.

1.-20. (canceled)
 21. An X-ray system for providing a plurality of X-raypulses of controlled dose per pulse, each of the plurality comprisingdifferent energy or different dose characteristics, or both, comprisinga linear accelerator having resonance for outputting pulses of highenergy electrons and configured to strike a target to thereby generate aplurality of X-ray pulses, a magnetron responsive to a first controlsignal for supplying RF energy pulses to the linear accelerator at aplurality of RF power levels with a corresponding plurality offrequencies, an electron gun driver, operating independently of themagnetron by being responsive to a second control signal different fromthe first control signal, for supplying to the linear accelerator pulsesof a plurality of beam currents at times substantially corresponding toan associated one of the RF energy pulses at a plurality of RF powerlevels, and wherein the magnetron output couples to the linearaccelerator at the plurality of RF power levels such that the energiesof X-ray pulses resulting from at least two of the plurality of pulsesof high energy electrons have controlled dose per pulse, at least one ofthe plurality of X-ray pulses is at a reduced dose relative to at leastone other of the plurality of X-ray pulses and the reduced dose pulsesare substantially within the range of 5× to 200× smaller than the pulsesthat are not reduced dose.
 22. The X-ray system of claim 21 wherein thebeam current pulses associated with the reduced dose X-ray pulses areshorter in at least one of duration and amplitude than the beam currentpulses associated with X-ray pulses that are not reduced dose.
 23. TheX-ray system of claim 21 wherein the beam current pulses associated withthe reduced dose X-ray pulses are shorter in both duration and amplitudethan the beam current pulses associated with X-ray pulses that are notreduced dose.
 24. The system of claim 21 wherein the plurality of X-raypulses forms a pattern of pulses ababab . . . ABABAB . . . , where “a”and “A” represent pulses of the same energy level but “a” is a reduceddose relative to “A” and “b” and “B” represent pulses of the same energylevel but “b” is a reduced dose relative to “B”.
 25. The system of claim21 wherein the plurality of X-ray pulses forms a pattern ccc . . .ABABAB . . . , where “c” represents pulses of a reduced dose anddifferent energy characteristics than either “A” or “B” pulses.
 26. Thesystem of claim 21 where the plurality of X-ray pulses forms a patternababab . . . ABABAB . . . , where “a” and “A” represent pulses ofdifferent energy levels but “a” is a reduced dose relative to “A”, and“b” and “B” represent pulses of different energy levels but “b” is areduced dose relative to “B”.
 27. The system of claim 21 wherein theplurality of X-ray pulses forms a pattern ccc . . . ABABAB . . . , where“c” represents pulses of a reduced dose and the same energycharacteristics “A” or “B” pulses.
 28. The X-ray system of claim 21wherein the reduced dose X-ray pulses are used to scan the passengercompartment of a vehicle and the X-ray pulses that are not reduced doseare used to scan the cargo compartment of a vehicle.
 29. The X-raysystem of claim 21 wherein the beam current pulses are supplied at timesapproximately corresponding with the associated RF energy pulses. 30.The X-ray system of claim 28 wherein the sum of the reduced dose pulsesduring a scan is substantially in the range of 0.25 microsieverts orless.
 31. The X-ray system of claim 21 wherein the electron gun driveris a dual grid gun driver.
 32. An X-ray system for providing X-raypulses of controlled dose per pulse comprising a linear acceleratorhaving an electron gun, a control system, an RF source responsive to thecontrol system to supply RF pulses to the linear accelerator, anelectron gun driver connected to the electron gun, responsive to thecontrol system but controlled independently of the RF source, forsupplying to the linear accelerator pulses of a plurality ofcontrollable beam currents at times substantially corresponding to RFpulses, the RF source and electron gun driver being configuredcooperatively to strike a target and thereby to cause the generation ofX-ray pulses of a first energy and first dose for scanning a firstregion, and to cause the generation of X-ray pulses of the first energyand a second dose for scanning a second region and wherein the firstdose is substantially in the range of 5× to 200× smaller than the seconddose.
 33. The X-ray system of claim 32 wherein the beam currentassociated with the reduced dose pulses is shorter in duration andamplitude than the beam currents associated with pulses that are notreduced dose.
 34. The X-ray system of claim 32 wherein the first regionis the passenger compartment of a vehicle and the second region is thecargo compartment of a vehicle.
 35. The X-ray system of claim 32 whereinthe plurality of pulses forms a pattern of pulses ccc . . . ABABAB . . ., where “c” represents pulses of a reduced dose but substantiallysimilar energy characteristics relative to at least one of “A” and “B”pulses.
 36. An X-ray system for providing X-ray pulses of controlleddose per pulse comprising a linear accelerator having an electron gun, acontrol system, an RF source responsive to the control system to supplyRF pulses to the linear accelerator, an electron gun driver connected tothe electron gun, responsive to the control system but controlledindependently of the RF source, for supplying to the linear acceleratorpulses of a plurality of controllable beam currents at timessubstantially corresponding to RF pulses, the RF source and electron gundriver being configured cooperatively to strike a target and thereby tocause the generation of X-ray pulses of a first energy and first dosefor scanning a first region, and to cause the generation of X-ray pulsesof a second energy and a second dose for scanning a second region andwherein the first dose is substantially in the range of 5× to 200×smaller than the second dose.
 37. The X-ray system of claim 36 whereinthe first dose X-ray pulses are associated with gun driver pulses havinga duration no more than one-fifth the duration of the gun driver pulsesassociated with the second dose X-ray pulses.
 38. The X-ray system ofclaim 36 wherein the first energy is substantially the same as thesecond energy.
 39. The X-ray system of claim 36 wherein the beam currentpulses occur at times substantially corresponding to the RF energypulses.
 40. The X-ray system of claim 36 wherein the electron gun driveris a dual grid gun driver.
 41. The X-ray system of claim 36 wherein thesum of the pulses during a scan of the first region totals less than0.25 microSieverts.